The transplantation of hormone- or protein-secreting cells from genetically non-identical members of the same species (i.e. allotransplantation) or from other species (i.e. xenotransplantion) is a promising strategy for the treatment of many diseases and disorders. Using alginate microcapsules to provide immunoisolation, hormone- or protein-secreting cells can be transplanted into a patient without the need for extensive treatment with immunosuppressant drugs. This principle has been successfully demonstrated by the transplantation of alginate-encapsulated pancreatic β-cells in diabetic rat models (Lim, F. and Sun, A. M. Science. 210, 908-910 (1980)). Methods of encapsulating biological material in alginate gels are described, for example, in U.S. Pat. No. 4,352,883 to Lim. In the Lim process, an aqueous solution containing the biological materials to be encapsulated is suspended in a solution of a water soluble polymer. The suspension is formed into droplets which are configured into discrete microcapsules by contact with multivalent cations such as Ca2+. The surface of the microcapsules is subsequently crosslinked with polyamino acids, forming a semipermeable membrane around the encapsulated materials.
The Lim method employs conditions which are mild enough to encapsulate cells without adversely affecting their subsequent survival and function. The resulting alginate microcapsules are semipermeable, possessing sufficient porosity to permit nutrients, waste, and the hormones and/or proteins secreted from encapsulated cells to diffuse freely into and out of the microcapsules, and, when implanted into an animal host, the alginate microcapsules effectively isolate the encapsulated cells from the host's immune system. See also U.S. Pat. No. 7,807,150 to Vacanti, et al.
Many other synthetic materials have been tried, including block copolymers such as polyethyleneglycol-diacrylate polymers, polyacrylates, and thermoplastic polymers, as reported by U.S. Pat. No. 6,129,761 to Hubbell and by Aebischer, et al, J Biomech Eng. 1991 May; 113(2):178-83. See Lesney Modern Drug Discovery 4(3), 45-46, 49, 50 (2001) for review of these materials.
Since Lim first reported on the transplantation of encapsulated cells, many other have tried to create “bioreactors” for cells that could maintain viability of the cells in the absence of vascularization, by diffusion of nutrients, gases and wastes through the encapsulating materials, and still protect the cells from the body's immune defenses against foreign cells and materials. Unfortunately, efforts to translate these therapies into human subjects have proven difficult. For example, alginate-encapsulated porcine islet cells transplanted into a human subject suffering from Type 1 diabetes initially demonstrated significant improvement and required decreased insulin dosing. However, by week 49, the patient's insulin dose returned to pre-transplant levels (Elliot, R. B. et al. Xenotransplantation. 2007; 14(2): 157-161).
In some cases, it is desirable to elicit fibrosis, for example, when the cells are implanted as a bulking material, as described in U.S. Pat. No. 6,060,053 and as subsequently approved by the Food and Drug Administration for treatment of vesicoureteral reflux.
The diminished efficacy of the implanted cells over time is the result of fibroblastic overgrowth of the alginate capsules. The alginate gel matrix provokes an inflammatory response upon implantation, resulting in the encapsulation of the alginate matrix with fibrous tissue. The fibrous tissue on the alginate capsule surface reduces the diffusion of nutrients and oxygen to the encapsulated cells, causing them to die. No better results have been obtained with the other materials.
Therefore, it is an object of the invention to provide polymers suitable for encapsulation and implantation of cells where the polymers have greater long term biocompatibility following implantation.
It is another object of the present invention to provide chemically modified, ionically crosslinkable alginates with improved biocompatibility and tailored physiochemical properties, including gel stability, pore size, and hydrophobicity/hydrophilicity.
It is also an object of the invention to provide methods for the encapsulation of cells using modified alginate polymers.
It is a further object of the invention to provide methods for treating a disorder or disease in a human or animal patient by transplanting exogenous biological material encapsulated in a modified alginate polymer.
Finally, it is an object of the invention to provide high-throughput methods for the characterization of modified alginate polymers.